Glass-ceramic materials have found utility in such varied products as cookware, tableware, missile nose cones, protective shields and industrial applications. Recently, an interest has arisen in producing a rigid, glass-ceramic disk as a substrate upon which a layer of magnetic media can be deposited. The ultimate product is an information disk to cooperate with a head pad in a magnetic memory storage device.
U.S. Pat. No. 2,920,961 (Stookey) originally disclosed the preparation of glass-ceramic articles through the heat treatment of precursor glass bodies by the following three general steps: (1) a glass forming batch, customarily containing a nucleating agent, is melted; (2) the melt is simultaneously cooled to a temperature below the transformation range of the glass and an article is shaped therefrom; and, (3) the resulting glass article is heat treated at temperatures above the annealing point of the glass and, frequently, above the softening point of the glass for a sufficient length of time to cause the glass to crystallize in situ. The heat treatment can be so scheduled as to control the size and, in some instances, the identity of the crystals developed. Thus, the crystallization present in a glass-ceramic article can be the result of both the base composition of the precursor glass and the heat treatment the glass body is subjected to.
Glass-ceramic articles containing a spinel-type crystal phase were originally disclosed in U.S. Pat. No. 3,268,315 (Stookey). This patent discloses a method for forming a glass-ceramic material consisting essentially, expressed in terms of weight percent on the oxide basis, of 40-70% SiO.sub.2, 14-34% Al.sub.2 O.sub.3, 8-27% MgO, and 0.4-2.5% Cr.sub.2 O.sub.3, with the total of these constituents constituting at least 95% by weight of the glass material, and subsequent glass-ceramic. The patent further discloses that the glassceramic exhibits a crystallization consisting essentially of at least one crystal phase selected from the group consisting of a magnesium metasilicate (enstatite) and a spinel. U.S. Pat. No. 4,687,749 (Beall) discloses glass-ceramic articles wherein enstatite constitutes the predominant crystal phase. These articles exhibit a high modulus of rupture, a use temperature in excess of 1200.degree. C., and a high fracture toughness. They consist essentially, expressed in terms of weight percent on the oxide basis, of about 20-35% MgO, 2-12% Al.sub.2 O.sub.3, 40-70% SiO.sub.2, and at least one metal oxide in the indicated proportions selected from the group consisting of 0-2% Li.sub.2 O, 0-4% CaO, 0-12% SrO and 0-17% BaO, at least 0.5% Li.sub.2 O being required when present alone and at least 1% SrO and/or BaO being required in the absence of Li.sub.2 O. The compositions include 5-15% TiO.sub.2 and/or ZrO.sub.2 for nucleation purposes.
Spinel-type crystal phase structures have also been disclosed in other patents such as, U.S. Pat. No. 3,873,329 (Beall); U.S. Pat. No. 3,936,287 (Beall et al.); U.S. Pat. No. 3,681,102 (Beall); U.S. Pat. No. 3,585,054 (Karstetter); U.S. Pat. No. 3,681,102 (Beall); U.S. Pat. No. 3,962,514 (Rittler); U.S. Pat. No. 4,059,454 (Reade); U.S. Pat. No. 4,867,750 (Pinckney); U.S. Pat. No. 5,028,567 (Gotoh et al.); and U.S. Pat. No. 5,079194 (Jean et al.). Also, U.K. Pat. No. 1,544,779 (Macmillan et al.).
One particularly desirable property of magnetic disk substrates is the ease with which such substrate can be fine polished. Information disks must be ultra-smooth to permit proper operation of a memory device. The difficulty in obtaining the desired ultra-smooth surface is a major concern in substrate production. One method which has been used in the past is to form glass-ceramic blanks into a desired shape, and then grind and polish the blanks to meet the smoothness requirements. This, of course, is a time-consuming, and hence expensive, operation.
U.S. Pat. No. 5,476,821 describes glass-ceramics having technical properties particularly well suited to producing an information disk substrate. As indicated above, these materials provide good fracture toughness and Knoop hardness values and a Young's modulus of 14-24.times.10.sup.6 psi, and capable of taking a fine polish. More recently, U.S. Pat. No. 5,491,116 disclosed a glass-ceramic article having a Mg-rich pyroxene and spinel-type crystal phases and a composition at least 92% of which consists essentially of 35-60% SiO.sub.2, 10-30% Al.sub.2 O.sub.3, 12-30% MgO, 0-10% ZnO, 5-20% TiO.sub.2, and 0-8% NiO.
The spinel- and enstatite+spinel-based glass-ceramics disclosed in the above two patents have been demonstrated to provide excellent substrates for rigid disk applications. Even though such materials are significantly easier to polish than those based predominantly on spinel, there is room for improvement in this area, and there continues to be a need for glass-ceramic materials with the same or improved technical properties, and which are also easy to polish.
Accordingly, it is the object of the present invention to provide modified glass-ceramics materials which provide the benefits of enstatite-spinel materials while providing greater ease of melting and polishing.